Voice coil actuators are electromagnetic devices which provide force proportional to current applied to a coil. U.S. Pat. No. 5,677,963, assigned to the assignee of the present application, is a example of such voice coil actuators, and incorporated by reference herein in its entirety.
Different configurations of linear voice coil actuators can provide different shapes of the Force vs. Stroke curves. However, these shapes cannot be changed without increasing the power level in the moving coil. But even this measure achieves only limited results due to a given distribution of the primary magnetic field and the armature reaction.
In a servo system, it may be necessary to maintain constant force or constant speed within a certain stroke. U.S. Pat. No. 5,177,383, entitled “Voice coil motor,” discloses coils with shorted turns employed in an attempt to achieve such characteristics. Because the turns in those coils are shorted, however, current in those turns cannot be adjusted when the moving coil moves from one position to another.